1. Field of the Invention
The invention relates to arrangements for determining the alcohol content of blood.
2. Description of the Prior Art
Such an arrangement is already known in which the concentration of alcohol in exhaled breath is measured in a measuring chamber having an inlet through which exhaled breath can be blown in, a radiation inlet at one end of the measuring chamber and a radiation outlet at the other end of the measuring chamber, a radiation source upstream of the radiation inlet, an interrupting device in the path of a radiation beam, a tuned photo-electric receiving device downstream of the radiation outlet, an indicator connected to the receiving device, and an absorption device which is displaceable in and at right angles to the path of the beam between the radiation outlet of the measuring chamber and the photo-electric receiver.
An arrangement of this type for analysing gases is already known (see FIG. 3 of British patent specification No. 1,113,986) which includes two measuring chambers, a reference gas being introduced into one of the measuring chambers and the gas to be analysed being introduced into the other measuring chamber. A monochromatic light beam produced by a laser is transmitted through the two measuring chambers. Different absorption occurs in the two measuring chambers. These different absorptions lead to different intensities at the photo-electric receivers arranged at the radiation outlets. A differential voltage is formed from these differing intensities. The differential voltage is indicated and is indicative of, for example, the concentration of the gas to be analysed relative to the reference gas. Before entering the measuring chambers, the monochromatic light beam produced by the laser is modulated by means of a mechanically operating interrupting device. This facilitates amplification of the voltages produced by the receivers. Wedge-shaped displaceable attenuators are located in front of the receivers in the two radiation paths. Upon admitting the reference gas into one of the measuring chambers, the indication can be increased or decreased, set to zero or calibrated by displacing these attenuators. Thus, the absorption devices serve for adjustment and calibration. The prerequisites for accurate operation of, and accurate indication by, this arrangement are constant conditions, independent of temperature, in the two measuring chambers, uniform linear behaviour of the two receivers, constant or identically varying working points of the two amplifiers etc. Furthermore, the known arrangement requires a monochromatic laser acting as the source of radiation. A laser of this type is very expensive.
According to Henry's law, a relationship exists between the alcohol content of the blood and the alcohol content of the breath. A one part per thousand concentration of blood alcohol corresponds to 1 mg of ethyl alcohol per cm.sup.3 of blood and leads to an alcohol content of 1 mg in 2100 cm.sup.3 of exhaled alveolar breath. Thus, the blood alcohol concentration can be determined by measuring the quantity of alcohol contained in the exhaled air. The alcohol molecules contained in air absorb in the infrared range a6 3.46 .mu.m. Thus, the quantity of alcohol in the breath and, correspondingly, the alcohol concentration in the blood can be determined by measuring the absorption at this wavelength. A known arrangement constructed in accordance with this principle includes a measuring chamber having an infrared radiator at one end and an infrared detector at its other end. A filter having a preferred band-pass width at 3.39 .mu.m is located in front of the detector. The indicator connected to the detector is set to zero by the pure air admitted into the measuring chamber. The radiation passing through the measuring chamber is damped at the said wave length when breath containing alcohol is subsequently blown into the measuring chamber. The indication correspondingly drops and thus is indicative of the alcohol concentration in the breath or in the blood. Accurate and reproducible indication requires constant operation, a constant working point and extremely strict linearity of the detector and of the amplifier connected to the output thereof. Differences in the operating behavior between measuring the pure air and the alcohol-containing breath subsequently blown into measuring chamber are not admissible. However, these conditions can only be fulfilled with a high technical expenditure. A further complication is the fact that the properties of an infrared are greatly dependent upon the ambient temperature. Also, the source of infrared radiation must produce radiation which is constant with respect to intensity and composition both when measuring with pure air and when subsequently measuring the alcohol-containing breath.
A further basic difficulty in determining the alcohol concentration by means of the known arrangement is that the antilog of the signal produced by the detector has to be taken. This can be done with only a limited degree of accuracy by electronic means. The necessity of taking the antilog follows from the Lambert/Beer Law. This law describes the absorption in a gas and reads as follows: EQU .phi..sub.e /.phi..sub.o = e.sup.-mcd
.phi..sub.o being the entering beam current and .phi..sub.e being the emerging beam current, m being a material constant, d being the length of the medium which is irradiated (length of path), and c being the gas concentration in question.
Thus, the known arrangement (see U.S. Pat. No. 3,792,272) has only limited accuracy of measurement.
A device for comparing the intensity of two beams is also known. This device includes, inter alia, a variable attenuating member in the path of the beam. This attenuating member is displaced by means of a servo-motor until the difference between the intensity of the two beams has been reduced to zero. A recording device then records the adjusting travel of the attenuating member as a measured value for the equalized difference between the intensities (see German Auslegeschrift No. 1,084,490).
Based on this prior art, the basic object of the invention is to provide an arrangement for determining the blood alcohol content such that the blood alcohol content can be reproducibly determined with great accuracy despite a low expenditure on circuitry.